FIG. 25 shows an example of a schematic configuration of a so called axial flow type gas laser oscillator. Referring to FIG. 25, the axial flow gas laser oscillator (hereinafter called AFGLO) is explained.
As shown in FIG. 25, the AFGLO is mainly composed of a laser resonator, a power supply unit 4, and a laser gas circulation part.
The laser resonator is composed of a discharge tube 1 having a discharge space 5, a rear mirror (hereinafter called RM) 6, and an output mirror (hereinafter called OPM) 7. The discharge tube (hereinafter called DT) 1 is composed of glass or other dielectric materials, and electrodes 2, 3 are disposed near both ends of the DT 1. In the DT 1 between the electrodes 2 and 3 the discharge space (hereinafter called DA) 5 is placed. The RM 6 and OPM 7 are disposed to enclose a plurality of DAs 5. The RM 6 is a reflector having a reflectivity of nearly 100%. The OPM 7 is a partial reflector, and a laser beam 8 is emitted from the OPM 7.
The power supply unit 4 is connected to the electrodes 2, 3 to perform a discharge in the DA 5.
The laser gas circulation part (hereinafter called LGCP) is composed of a blower 13, heat exchangers 11, 12, a laser gas passage 10, and a plurality of DAs 5 in DTs 1. The laser gas circulates in the LGCP composing the AFGLO in a direction of arrow 9. The blower 13 is for circulating the laser gas. By this blower 13, the flow velocity of laser gas is set at about 100 m/sec in the DA 5. The pressure in the LGCP is about 100 to 200 Torr. When a specified voltage is applied to the electrodes 2, 3 from the power supply unit 4, the DA 5 discharges. By this discharge and operation of the blower, the temperature of the laser gas elevates. The heat exchangers 11 and 12 are provided to cool the laser gas raised in temperature.
Described above is the configuration of the conventional AFGLO, and its operation is explained below.
The laser gas sent out from the blower 13 is guided into the DT 1 through the laser gas passage 10. In this state, when a specified voltage is applied to the electrodes 2, 3 from the power supply unit 4, the DA 5 discharges. The laser gas in the DA 5 obtains this discharge energy and is excited. The excited laser gas becomes a resonant state in the laser resonator formed of the RM 6 and OPM 7. As a result, a laser beam 8 is emitted from the OPM 7. The output laser beam 8 is used for laser machining or other application.
Problems of the conventional AFGLO are described below.
A first problem is described.
FIG. 26 shows a schematic configuration of the laser resonator including an optical bench of conventional AFGLO. The OPM 7 is held by an output mirror holder 150a. The RM 6 is held by a rear mirror holder 150b. On the other hand, the DT 1 is held by a discharge tube holder base (hereinafter called DT base) 170 which is an optical bench, by way of a discharge tube holder (hereinafter called DT holder) 160. Both ends of the DT base 170 are connected to corresponding mirror holders 150a, 150b. The mirror holders 150a, 150b, and DT base 170 are assembled to be in an unitary structure. The DT holder 160 and mirror holders 150a, 150b are connected through a connection tube 180 with both ends held with O-rings or the like so as to be slidable.
In this configuration, the axis linking the center of the RM 6 and the center of the OPM 7, and axes of the RM 6 and OPM 7 are disposed to be vertical to each other. That is, the RM 6 and OPM 7 are disposed parallel to each other. The parallelism is adjusted to a precision of several μm or less to each other. The axis linking the centers of the RM 6 and OPM 7 is disposed to coincide with the central axis of the DT 1.
To obtain a normal laser output, following conditions are required;                the parallelism of RM 6 and OPM 7 of 10−6 radian or less, and        the precision of the axis formed by the mirror and the axis formed by the DT is tens of μm or less.        
To maintain this precision, the mirror holders and the DT base are formed in an unitary rigid structure.
In the conventional laser oscillator having such configuration, the first problem is explained.
The degree of vacuum in LGCP is about 100 to 200 Torr. On the other hand, its outside is an atmospheric pressure (760 Torr). Between the inside and outside of the LGCP, a stress due to the pressure difference (hereafter called vacuum force) is applied. Usually, both ends of the DT base 170 are held by a support structure (not shown). The LGCP is also held by a support structure (not shown). Therefore, in the DT holder 160c in the central part shown in FIG. 25, a downward stress is applied due to the pressure difference.
The DT base 170 is made of material of high rigidity such as steel so as not to be bent by the stress due to such pressure difference. To maintain the rigidity, the DT base 170 has a considerably large structure as compared with other parts such as DT 1.
For the pourpose of increasing the rigidity, however, the size is limited. Therefore, by the vacuum force, the DT base 170 may be bent by about tens of μm. As described above, the DT base 170 and mirror holders 150a, 150b are assembled in an unitary structure. Accordingly, if the DT base 170 is bent only by tens of μm, the parallelism between the mirror holder 150a and mirror holder 150b is changed. By this change in parallelism, the laser output may be lowered.
Besides, since the thermal capacity of the DT base 170 is large, if ambient temperature varies, it cannot follow up the temperature changes. Due to change in ambient temperature, a temperature difference may occur in the parts of the DT base 170 (for example, temperature difference between upper part and lower part, or temperature difference between right side and left side, as shown in FIG. 26). If a temperature difference occurs, the DT base 170 is bent due to a thermal expansion or a thermal shrinkage. As a result, the parallelism of RM 6 and OPM 7 cannot be maintained. By this change in parallelism, the laser output may be lowered. FIG. 27 schematically shows the change in the laser output depending on the ambient temperature.
To address this problem, hitherto, the following measures were taken.
As a measure against bending of DT base 170 by vacuum force, for example, it was attempted to use a canceler for canceling the stress due to pressure difference in order to keep balance of stress due to pressure difference between inside and outside. However, the canceler generated an unexpected stress, and produced adverse effects.
On the other hand, as a measure against the expansion and the shrinkage due to the temperature difference, it was attempted to control the DT base 170 at a constant temperature. This attempt is intended to pass liquid (for example, water) in the DT base 170, and control the liquid temperature to remain constant. However, the volume of the DT base 170 is large in order increase the rigidity. Therefore, the thermal capacity of the DT base 170 becomes larger, and the temperature difference cannot be completely eliminated.
A second problem of the conventional AFGLO is as follows.
The flow of laser gas in the DT 1 is preferred to be uniform in the gas flow direction as far as possible from entry of gas in the DT 1 until its exhaust. When the gas flow is uniform, the discharge state is stable. As a result, the efficiency of laser output versus an electric input to the DA 5 is enhanced (known as a laser oscillation efficiency). Owing to the specific configuration of the AFGLO, the structure is complicated if the laser gas lead-in portion is provided coaxially with the DT 1. Actually, as shown in FIG. 28, the laser gas lead-in portion is generally disposed nearly at right angle to the DT 1. FIG. 28 and FIG. 29 schematically show the gas flow in the DT 1. FIG. 29 is a sectional view along line 29—29 in FIG. 28. In this configuration, as shown in FIG. 28, in the DT 1, particularly near the laser gas inlet 137, a vortex 136 is likely to occur in the gas flow. By the vortex, the gas flow in the DT is disturbed. As a result, the laser oscillation efficiency cannot be enhanced. FIG. 30 shows the relation between electric input in the DA 5 and laser output.
As proposed in a prior art (Japanese Patent Laid-Open Publication No. 7-142787), a chamber is provided for storing the gas temporarily, and it is connected to the laser gas lead-in portion. By eliminating the directivity of laser gas entering the laser gas lead-in portion, it is intended to eliminate a non-uniformity of gas flow in the DT. According to the study by the present inventors, the gas flow becomes not uniform when feeding gas is led into the DT from the laser lead-in portion, and vortices were formed in various portions. As a result, the laser oscillation efficiency could not be further enhanced by the configuration proposed in Japanese Patent Laid-open Publication No. 7-142787.
Further, the conventional AFGLO has a third problem.
When the voltage between the electrodes 2, 3 provided around the DT 1 reaches a discharge start voltage, discharge starts. At this discharge start moment, a large rush current flows into the DT 1. When discharge current starts to flow, the impedance of DT drops, and soon at maintenance voltage of about 20 kV settles. In this state, the current is stable and a uniform discharge is obtained. However, due to rush current at the discharge start moment, the discharge is disturbed temporarily. It takes a certain time until the discharge is stabilized. The value of the rush current is proportional to the discharge start voltage. It is hence important to lower the discharge start voltage in order to stabilize discharge.
In a prior art, as shown in FIG. 31, an auxiliary electrode 156 is disposed near the electrode 2 in the DT 1, and the auxiliary electrode 156 and the electrode 3 are connected with a high resistance resistor 158 of several MΩ. In this case, since the distance between the auxiliary electrode 156 and electrode 3 is too long, if laser gas is ionized between the auxiliary electrode 156 and electrode 2, it is almost recombined before reaching the electrode 3. Therefore, in this configuration, notable effect for decreasing the discharge start voltage cannot be obtained.
FIG. 32 shows another typical example of prior art. Along the outer surface of the DT 1, a conductor 159 is extended from the electrode 2 to the electrode 3 side, and an auxiliary electrode 156 is attached to the end of the conductor 159 closer to the electrode 3 side. The auxiliary electrode 156 is attached to the outer surface of the DT 1 via an insulating sheet 162 made of a dielectric material. To lower the discharge start voltage, it was attempted to reduce the thickness of the dielectric materials, but holes were formed in the wall of the DT 1 due to micro discharge in a course of time.
Thus, in the conventional AFGLO, usually, a mechanism called auxiliary electrode was added. It is an attempt to lower the rush current upon start of discharge by lowering the insulation breakdown voltage in the DT by some mechanism so as to ignite discharge easily. The auxiliary electrode itself was a good idea, but none of the prior arts was satisfactory in the aspects of performance and reliability.
Summing up, in the conventional AFGLO,
1) Stress occurs in the parts of the resonator due to pressure difference between the LGCP and the outside at atmospheric pressure. By this stress, the DT base 170 may be bent by about tens of μm. Since the DT base 170 and a pair of mirror holders 150 are in unitary structure, if the DT base 170 is bent only by tens of μm, the mutual angle of the pair of mirror holders 150a and 150b is varied. As a result, it was difficult to enhance the stability of laser output further.
2) In the DT, the laser gas flow tends to be not uniform in the central part or in peripheral part of the DT. As a result, uniform gas flow is not realized. Hence, the energy efficiency could not be enhanced further.
3) A large rush current flows in the DT at the discharge start moment when the voltage between the electrodes 2, 3 reaches the discharge start voltage. When the rush current flows at the discharge start moment, a large current flows, and the discharge is disturbed temporarily. Accordingly, it takes some time until discharge is stabilized, and discharge is unstable in this period (that is, the laser output is unstable). This transient unstable period of discharge cannot be shortened.